1. Field of the Invention
The present invention relates to a developing device included in a copier, facsimile apparatus, laser printer or similar electrophotographic apparatus. More particularly, the present invention is concerned with a developing device using a developer consisting of toner and magnetic carrier, and a developing roller therefor.
2. Discussion of the Background
It is a common practice with a copier or similar electrophotographic image forming apparatus to electrostatically from a latent image on a photoconductive element, or image carrier, and develop it with a developer stored in a developing device. The developing device includes a developing roller for conveying a developer consisting of toner and magnetic carrier to the photoconductive element and returning, after the deposition of the toner on the element, the carrier and excess toner to a developer storing section included in the casing of the developing device. To allow the developing roller to so operate, it has been customary to form a flux density pattern on the outer periphery of a sleeve forming a part of the developing roller. The flux density pattern is formed by a main pole for development, a plurality of conveying poles, a releasing pole, and a depositing pole.
In the electrophotographic image forming apparatuses art, there is an increasing demand for miniaturization and high-speed operation allowing a great amount of information to be rapidly dealt with in a limited space. To meet this demand, it is necessary to reduce the size of the developing device operable with the toner and carrier mixture and the size of its developing roller as far as possible.
However, the developing roller with the flux density pattern mentioned earlier has the following problem left unsolved. When the diameter of the developing roller is reduced for the miniaturization of the developing device, the actual distance on the sleeve decreases although the angle (width) may remain the same. As a result, a part of the developer on the sleeve is apt to move from the releasing pole to the depositing pole together with the sleeve before it is fully released from the sleeve, lowering image density and therefore image quality. This is particularly true when the outside diameter of the developing roller or sleeve is less than 20 mm. Further, the conventional process speed causes the sleeve to rotate at a higher speed, aggravating the above occurrence.
In light of the above, Japanese Patent Laid-Open Publication No. 9-281794, for example, teaches a developing roller on which all the magnetic poles from the conveying pole to the depositing pole are of the same polarity. In this configuration, a repulsive magnetic field is formed for exerting a magnetic force which releases the developer from the sleeve. With such a developing roller, it is possible to promote the sharp release of the developer from the roller even when the roller has a small diameter. It was experimentally found that when the poles from the conveying pole to the depositing pole were of the same polarity, the above developing roller was satisfactory as to the initial image characteristic. However, the developing characteristic and therefore development .gamma. (gamma) characteristic varied when a number of images were output, depending on the flux density pattern. As a result, a greater amount of toner was deposited on the photoconductive element, bringing about defective images due to the scattering of toner around an image and the offset to a fixing roller.